1. Field of the Invention
The present invention relates to a coil spring for vehicle suspension used for a suspension mechanism of a vehicle such as an automobile or the like, excellent in corrosion durability, and method for manufacturing the same.
2. Description of the Related Art
It is known that a coil spring for suspension mechanism is subject to an influence of an antifreeze agent scattered on a road in the wintertime or the like. The antifreeze agent contains salt, and hence the agent promotes corrosion of the surface of the coil spring constituted of spring steel. It is known that a corrosion pit (etch pit) particularly exerts a great influence upon the durability of the coil spring. For example, as shown in FIG. 11, part of the surface la of the spring wire 1 is corroded to form a hole-like shape by moisture or salt, whereby a corrosion pit 2 is formed. Although the shape of the corrosion pit 2 varies, there is even a corrosion pit 2 having a cross section as schematically shown in, for example, FIG. 12.
As shown in FIG. 13, when the corrosion pit 2 reaches a depth R of a certain level or more, and the weight of the vehicle is continuously applied to the coil spring, a fatigue crack 3 occurs at the bottom part 2a or the like of the pit 2. When the crack 3 grows large, the coil spring is broken. In other words, even when the coil spring is used in a corrosive environment, the coil spring can be used without breakage if the size of the corrosion pit is within the permissible pit depth Rmax.
On the other hand, it is desired that the coil spring for suspension be used at higher stress in order to effect weight reduction of a vehicle. In order to realize the higher stress in the coil spring, it becomes important to impart compressive residual stress to the vicinity of the surface of the spring wire. It is known from the past that by subjecting a coil spring to shot peening, compressive residual stress is imparted to the vicinity of the surface of the coil spring, and the durability thereof is enhanced. For example, in Jpn. Pat. Appln. KOKAI Publication No. 2000-345238 or Jpn. Pat. Appln. KOKAI Publication No. 2008-106365, multistage shot peening is disclosed. In the multistage shot peening, shot peening is carried out a plurality of times in a dividing manner.
As means for producing compressive residual stress in a region from the surface of the spring to a deep position, stress peening and warm peening (hot peening) are known. In the stress peening, the shot is applied to the coil spring in a state where the spring is compressed. In the warm peening, the shot is applied to the coil spring in a state where the spring is heated at a temperature of about 250° C. In comparison with the ordinary shot peening to be carried out at room temperature, in the stress peening or warm peening, it is possible to cause compressive residual stress to appear in a region up to a deep position in the material. However, the stress peening requires equipments configured to compress the coil spring. Furthermore, in the stress peening, the shot is applied to the coil spring in a state where the coil spring is compressed, and hence gaps between spring wire parts become small. Accordingly, there is a problem that it is hard for the shot to hit the inside of the coil spring or positions between the spring wire parts.
In a conventional coil spring for suspension, large compressive residual stress is imparted to the vicinity of the surface of the spring by shot peening. However, when the coil spring for suspension is used in a corrosive environment in which a corrosion pit occurs, the coil spring breaks in some cases after a relatively short period of use by the growth of the corrosion pit. Thus, it is also proposed to improve the corrosion resistance, and suppress occurrence of the corrosion pit and growth of the corrosion pit by contriving the components of the material (spring steel).
For example, the corrosion resistance of the coil spring is improved by adding an alloy element such as Ni, Cr, Mo or the like to the spring steel. However, the spring steel containing such an alloy element is expensive, this being a cause making the cost of the coil spring high. Further, once the size of the corrosion pit reaches the permissible pit depth, there is the possibility of the coil spring being broken while making a fatigue crack occurring at a bottom part or the like of the corrosion pit a starting point.